Provider link state bridging (plsb) computation method

ABSTRACT

A method of multicast route computation in a link state protocol controlled network. A spanning tree is computed from a first node to every other node in the network using a known spanning tree protocol. The network is then divided into two or more partitions, each partition encompassing an immediate neighbour node of the first node and any nodes of the network subtending the neighbour node on the spanning tree. Two or more of the partitions are merged when a predetermined criterion is satisfied. Nodes within all of the partitions except a largest one of the partitions are then identified, and each identified node examined to identify node pairs for which a respective shortest path traverses the first node.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the first application filed for the present invention.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present invention relates to traffic forwarding in packet networks,and in particular to a Provider Link State Bridging (PLSB) computationmethod.

BACKGROUND OF THE INVENTION

Network operators and carriers are deploying packet-switchedcommunications networks in place of circuit-switched networks. Inpacket-switched networks such as Internet Protocol (IP) networks, IPpackets are routed according to routing state stored at each IP routerin the network. Similarly, in Ethernet networks, Ethernet frames areforwarded according to forwarding state stored at each Ethernet switchin the network. The present invention applies to communications networksemploying any Protocol Data Unit (PDU) based network and in thisdocument, the terms “packet” and “packet-switched network”, “routing”,“frame” and “frame-based network”, “forwarding” and cognate terms areintended to cover any PDUs, communications networks using PDUs and theselective transmission of PDUs from network node to network node.

Multicast forwarding of data packets (where packets are sent from asource node to multiple destination nodes more or less simultaneously)is of increasing importance as demand for services such as InternetProtocol Television (IPTV) and Video on Demand (VoD) grows.

Protocols such as Intermediate System-Intermediate System (IS-IS) andOpen Shortest Path First (OSPF) and Multicast OSPF are used todistribute topology information to permit distributed calculation ofpaths that interconnect multiple nodes, resulting in the installation ofthe forwarding state required to implement those paths. OSPF and IS-ISare run in a distributed manner across nodes of the network so that, forexample, when a topology change occurs in the network such as a node orlink failure, this information is flooded to all nodes by the protocol'soperation, and each node will locally recompute paths to circumvent thefailure based on a consistent view of the network topology.

In Ethernet networks, Provider Backbone Transport (PBT), also known asProvider Back-Bone Bridging-Traffic Engineering (PBB-TE), as describedin Applicant's British patent number GB 2422508 is used to provide aunicast Ethernet transport technology. Provider Link State Bridging(PLSB) as described in Applicant's co-pending U.S. patent applicationSer. No. 11/537,775 will be used to provide a multicast transportcapability for Ethernet networks using IS-IS to set up both unicastpaths and multicast trees in the network. Both above patent documentsare hereby incorporated by reference.

While the present invention is not limited to the application of arouting system to Ethernet bridging, Ethernet terminology is used inthis disclosure where possible. So, for example, the term filteringdatabase (FDB) can be considered interchangeable with any term for aninformation repository of packet forwarding information, such asforwarding information base or label information base.

Typically, multicast trees in a PLSB network are computed using anall-pairs shortest path multicast route computation algorithm known, forexample, from Applicant's co-pending U.S. Patent Application PublicationNo. 20070165657. In accordance with this method, when a node receiveseither a multicast group membership change or a network topology change(for example via a Link State Protocol Data Unit—LSP) the node employsalgorithms such as Dijkstra's algorithm to compute both unicastconnectivity and the set of pairs of network nodes that are connected bya shortest path which traverses the computing node. For that set of nodepairs, the node determines where intersections of multicast groupmembership occur, and defines the required FDB entries to instantiateits portion of multicast paths accordingly. Both Unicast and Multicastforwarding state implementing the computed paths is then installed inthe node's filtering database (FDB), so that received packets can beforwarded to the appropriate output port(s) of the node, based on thedestination address in the frame.

As may be appreciated, identifying pairs of nodes for which therespective shortest path traverses a particular node is computationallyintensive, because it involves examining the paths extending from eachnode to every other node. In some cases, the challenge of performing therequired computations within an acceptable period of time can imposelimitations on the size of the network. Clearly, more powerfulprocessors can be used to increase the speed of computation, but only byincreasing the cost of each node, which may be undesirable.

Techniques for improving the efficiency of multicast route computationin packet switched networks remain highly desirable.

SUMMARY OF THE INVENTION

Thus, an aspect of the present invention provides a method of multicastroute computation in a link state protocol controlled network. Aspanning tree is computed from a first node to every other node in thenetwork using a known spanning tree protocol. The network is thendivided into two or more partitions, each partition encompassing animmediate neighbour node of the first node and any nodes of the networksubtending the neighbour node on the spanning tree. Two or more of thepartitions are merged when a predetermined criterion is satisfied. Nodeswithin all of the partitions except a largest one of the partitions arethen identified, and each identified node is examined to identify nodepairs for which a respective shortest path traverses the first node.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a flow chart illustrating principle steps in a methodaccording to a representative embodiment of the present invention;

FIGS. 2 a-e illustrate steps in the process of FIG. 1 implemented in arepresentative network.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a PLSB computation method in which thenumber of nodes that must be examined, in order to find all of the nodepairs for which the respective shortest path traverses a given node, isminimized. In some cases, the number of nodes that must be examined canbe reduced to zero. Embodiments of the invention are described below, byway of example only, with reference to FIGS. 1-2 e.

As an initial matter, it should be noted that the method of the presentinvention is valid for networks in which computed shortest paths aresymmetric (that is, the network can be represented as an undirectedgraph) and, if two or more equal-cost paths can be computed between anytwo nodes, a tie-breaking method must be implemented which will selectone of the equal-cost paths in such a way that the selected “shortest”paths are symmetric and locally consistent. In this respect, “locallyconsistent” means that any sub-path of the equal-cost path selected bythe tie-breaking method must itself be a shortest path selected by thetie-breaking method. A representative tie-breaking method, which may beused in conjunction with the methods of the present invention, is knownfrom Applicant's co-pending U.S. patent application Ser. No. 11/964,478,which was filed on Dec. 26, 2007.

FIG. 1 is a flow chart illustrating principle steps in a methodaccording to a representative embodiment of the present invention, andFIGS. 2 a-e illustrate steps in the process of FIG. 1 implemented in arepresentative network.

Referring first to FIG. 2 a, a representative PLSB network comprises aplurality of nodes (labelled as nodes A-R) interconnected by links. Asis typical in a PLSB network, every node in the network of FIGS. 2 a-eis connected to at least two other nodes, although this is notessential. Preferably, the method of the present invention will beimplemented to execute in every node, substantially in parallel. In thefollowing description, the method is described by way of an example inwhich shortest paths traversing node “A” are identified.

Referring now to FIGS. 1 and 2 b, at a first step, a spanning tree fromnode “A” to every other node in the network is computed, for exampleusing a known shortest path tree algorithm such as Dijkstra's algorithm.As may be seen in FIG. 2 b, the spanning tree (indicated by bold linesin FIGS. 2 b-e) comprises a plurality of branches extending from node“A” to each of its immediate neighbours (nodes B, C, D and E). Byconstruction, all of the nodes of the network are on one of thesebranches. Thus it is possible to notionally divide the network into aset of partitions, each of which encompasses a respective branch of thespanning tree.

By construction, each partition will therefore include a respective oneof the neighbour nodes, and all of the nodes that subtend that neighbournode on the spanning tree. For convenience of description, eachbranch/partition may be referenced using the identifier of therespective neighbour node which serves as the root of that branch. Thus,in FIG. 2 c, the four partitions are identified as partitions “B”, “C”,“D” and “E”, following the identity of their respective root nodes.

As may be seen from FIG. 2 c, any shortest path that traverses node “A”must necessarily originate in one partition and terminate in a secondpartition. Because of path symmetry between node pairs, the number ofnodes that need to be examined, in order to find all of the node pairsfor which the respective shortest path traverses node “A”, can bereduced by considering the nodes in all but one of the partitions. Thebenefits of this reduction can be maximized by recognising that thelargest partition, in terms of the number of member nodes, can beselected as the omitted partition, so that only paths terminating atnodes in the remaining smaller partitions are considered.

A further reduction in the number of nodes that must be examined can beobtained by recognising that the shortest path between any node in onepartition and any node in any other partition will not traverse node “A”if, and only if, there is a path between the respective root nodes ofthe involved partitions that is shorter than the two-hop path throughnode “A”. For example, consider a path between nodes M and R in theembodiment of FIG. 2, which are respectively located in partitions “D”and “E”. In this example we will only consider number of hops as theshortest path criterion although other criteria are equally applicable.Inspection of the network reveals that the root nodes “D” and “E” aredirectly connected by a link. Consequently, the shortest path betweennodes M and R traverses only root nodes “D” and “E”, and does nottraverse node “A”. Consideration of the other nodes in partitions “D”and “E” reveals that, while not all of the shortest paths (between anode in partition “D” and a node in partition “E”) traverse the directlink between root nodes “D” and “E”, the presence of that direct linkguarantees that none of these shortest paths will traverse node “A”.Accordingly, partitions “D” and “E” can be merged into a singlesuper-partition “DE” for the purpose of the computations at “A”.

This process of identifying shorter paths between pairs of root nodes inthe pair of partitions or super-partitions under consideration, andmerging partitions whenever a sufficient number of shorter paths isfound, can be repeated until either: all of the partitions have beenmerged into a one super-partition (which in fact encompasses the entirenetwork except node “A”); or there are no more pairs of partitions forwhich all the root nodes are interconnected by paths that are shorterthan the two-hop paths through node “A”. Whether two partitions can bemerged can be determined by considering the root nodes in the proposedmerged partition. In the simple case of a pair of partitions each havinga single root node, the two partitions can be merged if, and only if,the respective root nodes of the two partitions are connected by ashorter path than the two-hop path through node “A”. For the morecomplex case of a partition having one root node and a super-partitionhaving N (where N>1) root nodes, the two partitions can be mergedtogether if none of the shortest paths between the root node of thepartition and the N root nodes of the super-partition go through node“A”.

Thus, continuing the example at FIG. 2 d, and continuing to use thenumber of hops as shortest path criteria, the partition “B” can bemerged with super-partition “DE” (having N=2 root nodes) to producesuper-partition “BDE”, because root node “B” is directly connected tothe two root nodes of super-partition “DE”. This guarantees that thereis no shortest path between any pair of nodes in super-partition “BDE”which traverses node “A”. On the other hand, and referring to FIG. 2 e,partition “C” can not be merged with super partition “BDE”, becausethere is no direct link between root node “C” and root node E of thesuper partition “BDE”. In fact, partition “C” could not be merged withpartition “E” or any super-partition including partition “E”.

As may be seen in FIG. 2 e, completion of the above process of mergingpartitions results in the network being divided into two partitions,namely partition “C” and super partition “BDE”. As noted above, all ofthe shortest paths of interest can be found by examining the nodeswithin each partition except the largest partition. In the case of FIG.2 e, all of the shortest paths that traverse node “A” can be discoveredby examining node “C” to identify each shortest path extending from node“C” which traverses node “A”. As will be appreciated, this dramaticallyreduces the PLSB computation required for the network in this example,since paths extending from only one node (as opposed to 17 nodes usingconventional methods) must be considered.

It will be understood that the benefits obtained by merging partitionsis dependent on the network topology. In a scenario in which allpartitions can be merged into a single super-partition, which thereforeencompasses the entire network, then the number of nodes that must beexamined is zero (after the initial overhead of the partitioning step).In a more typical scenario, the process of merging partitions willresult in a plurality of partitions and/or super-partitions. In thespecial case in which node “A” is a dual-connected edge node, theinitial number of partitions is two. If these two partitions can bemerged successfully, the number of nodes that must be subsequentlyexamined is reduced to zero. Otherwise, in the worst case, the number ofnodes that must be examined is slightly less than half the number ofnodes in the network, which is still a substantial improvement overconventional methods.

As is known in the art, route computation methods such as IntermediateSystem-Intermediate System (IS-IS) and Open Shortest Path First (OSPF)and Multicast OSPF can produce multiple equal-cost paths between nodepairs. In such cases, the above-described method for merging partitionsmay be used without modification where either: the “cost” of each pathis proportional to the number of hops; or the “cost” of a direct linkbetween two partitions is less than the “cost” of the two-hop paththrough the node of interest (node “A” in the example of FIG. 2).

A tie-breaking algorithm must be used to select a “shortest” path orsubset of shortest paths from among the set of equal-cost paths betweena node pair. In such cases, the above-described method for mergingpartitions can be used, provided that the “shortest” path(s) selected bythe tie-breaking algorithm are symmetric and locally consistent.

For example, in the network of FIG. 2, consider a scenario in which theroute computation methods yields three equal cost paths between nodes Cand E, and a tie-breaking algorithm is used to select one of theseequal-cost paths as the “shortest” path. In this case, theabove-described methods could be used to merge partition “C” withsuper-partition “BDE” if the tie-breaking method selected, as theshortest path, either one of the two paths through nodes B or D, but notthe path through node “A”.

As may be appreciated, this same methodology can be extended to a casewhere the route computation algorithm computes a set of equal-costpaths, and the tie breaking mechanism operates to select a subset of twoor more of these equal cost paths as the shortest paths. In this case,the criterion for merging two partitions is that none of the selectedshortest paths traverse the node under consideration. Thus, for example,the tie-breaking mechanism could potentially select any two of the pathsbetween nodes C and E as the set of shortest paths, and theabove-described methods could be used to merge partition “C” withsuper-partition “BDE” if this set of shortest paths did not include thepath through node “A”.

The embodiment(s) of the invention described above is(are) intended tobe exemplary only. The scope of the invention is therefore intended tobe limited solely by the scope of the appended claims.

1. A method of multicast route computation in a link state protocolcontrolled network, the method comprising steps of: computing a spanningtree from a first node to every other node in the network using a knownshortest path tree algorithm; dividing the network into partitions, eachpartition encompassing an immediate neighbour node of the first node onthe computed spanning tree and any nodes of the network subtending theneighbour node on the computed spanning tree; merging two or more of thepartitions when a predetermined criterion is satisfied; examining nodeswithin all of the partitions except a largest one of the partitions toidentify node pairs for which a respective shortest path traverses thefirst node.
 2. The method as claimed in claim 1, wherein each one of afirst partition and a second partition includes a respective one of theneighbour nodes, and wherein the predetermined criterion is that ashortest path between each of the included neighbour nodes does nottraverse the first node.
 3. The method as claimed in claim 2, whereinthe shortest path is a direct link.
 4. The method as claimed in claim 2,wherein the shortest path is selected, from a set of two or more equalcost paths between each of the involved neighbour nodes, by atie-breaking method which is symmetric and locally consistent.
 5. Themethod as claimed in claim 1, wherein a first partition comprises arespective one of the neighbour nodes and a second partition is asuper-partition comprising two or more of the neighbour nodes, andwherein the predetermined criterion is that respective shortest pathsbetween the one neighbour node of the first partition and the two ormore neighbour nodes of the second partition, do not traverse the firstnode.
 6. The method as claimed in claim 5, wherein at least one of theshortest paths is a direct link.
 7. The method as claimed in claim 5,wherein at least one of the shortest paths is selected from a set of twoor more equal cost paths between the one neighbour node of the firstpartition and one of the two or more neighbour nodes of the secondpartition by a tie-breaking method which is symmetric and locallyconsistent.
 8. The method as claimed in claim 1, wherein a firstpartition comprises a respective one of the neighbour nodes and a secondpartition is a super-partition comprising two or more of the neighbournodes, the one neighbour node of the first partition being connected toeach of the two or more neighbour nodes of the second partition by arespective set of one or more shortest paths, at least one set ofshortest paths comprising two or more equal cost paths selected by atie-breaking method which is symmetric and locally consistent, andwherein the predetermined criterion is that none of the two or moreequal cost paths within any given set of shortest paths traverses thefirst node